| The PMN-PT-based relaxor ferroelectrics are a key material for making piezoelectric devices owing to their excellent piezoelectric and electromechanical properties.Ferroelectric materials consist of inherent spontaneous polarizations,and small regions with aligned polarization called domains.Therefore,it is believed that the domains have a very important effect on electrical characteristics of ferroelectrics.Studying the electrical characteristics and domain structures of PMN-PT-based relaxor ferroelectrics is important to understand the effect of domain structures on the dielectric relaxation behavior,piezoelectricity and phase transition,and to develop the ferroelectric materials with high piezoelectric property.This thesis aims to explore the mechanism of piezoelectric property and domain structures of PIN-PMN-PT relaxor ferroelectric single crystal with excellent piezoelectric property,high temperature and electric field stability.And then to prepare the PMN-PT transparent ceramic with high piezoelectricity by doping rare earth elements to control the domain structures,which provides the necessary material for the producing transparent ultrasonic transducers,and then deeply explores the mechanism of the domain structures and the degree of disorder local polarization orientation on the high piezoelectricity of PMN-PT transparent ceramic.At present,there are still several scientific issues need to be clarified.Firstly,the relationship between the domains and dielectric relaxation behavior,piezoelectric property is still not well understood.Furthermore,the formation process of the “4R” engineering domain structures of PINPMN-PT relaxor ferroelectric single crystal with R phase.Moreover,the low piezoelectric property of ferroelectric transparent ceramics is difficult to meet the requirements of transparent ultrasonic transducers.In view of the above problems,the main research contents of this paper are as follows:Firstly,by studying the dielectric temperature spectrum,P-E and S-E loops of PINPMN-x PT single crystals as well as the domains,the degree of disorder local polarization orientation and the dynamics of local polarization rotation under the PFM tip DC voltage,the mechanism of the domains on the dielectric relaxation characteristic,electric stability and piezoelectricity is deeply explored.The results show that the disordered “island”nanodomains with irregular shape and small size are the main reason for the excellent piezoelectricity of the samples.And it is also determined that the 90? domains help to enhance the electric field stability for PIN-PMN-x PT single crystals.In addition,the“island”-like domains will turn into “matrix”-like domains,“strip”-like domains and“human”-shaped “4R” engineering domain structures when the poling electric fields applied in PIN-PMN-x PT single crystals.Then the effects of the poling electric fields on the structural phase transition,piezoelectric and electromechanical properties,ans domain structures of PIN-PMN-27 PT single crystal with R phase.It is found that the single crystal begins to show macroscopic piezoelectric and electromechanical properties,as well as the structural phase transition from normal ferroelectric phase to relaxor ferroelectric phase when the single crystal has a locally ordered domains.Furthermore,the piezoelectric constant and electromechanical coupling coefficient increases sharply,and the phase transition from R phase to T phase begins to appear when most regions of the single crystal has a long-range ordered domain structures.When there is a stable and long-range ordered “4R” engineering domains in the whole single crystal,the piezoelectric constant and electromechanical coupling coefficient will reach the maximum value.And at this time,the structural phase transition from normal ferroelectric phase to relaxor ferroelectric phase and R phase to T phase of the single crystal remain unchanged.Based on the relationship between domain structures and piezoelectricity of PINPMN-x PT single crystal,the irregular shape,small size and highly disordered nanosized domain structures which helps to improve the piezoelectricity are controlled by rare earth element Eu doping in PMN-25 PT ferroelectric transparent ceramics.And the high piezoelectricity with a piezoelectric coefficient of 850 p C/N are achieved in the 2 mol%Eu-doped PMN-25 PT ferroelectric transparent ceramic,which has reached the maximum value of piezoelectric coefficient among the transparent ferroelectric ceramics reported in the current references.Moreover,by studying the domains and the degree of local polarization orientation disorder of the Eu-doped ceramics,we find that Eu doping significantly enhanced the local structural heterogeneity of PMN-25 PT transparent ceramics,and then the nanodomains with irregular shape and disordered distribution were formed,which promotes the dynamic behavior of local polarization rotation of the nanosized domain structures,thereby enhancing its piezoelectric performance. |
PDF Full Text Request